Spread the squash and onion slices in the baking dish, dot with 1/4 cup of butter cut into chunks, cover the dish, and microwave on High 5 to 7 minutes, or until the vegetables are steaming hot and tender.

In a bowl, whisk together milk and cornstarch until smooth, and stir in the beaten eggs, salt, and pepper. Pour the milk mixture over the squash and onions.

Melt 1/4 cup butter.

Sprinkle the corn flakes cereal evenly over the top of the casserole, drizzle with the melted butter, and bake in preheated oven for 25 to 30 minutes, until the cereal topping is golden and crisp.